Microwave level gauge, or radar level gauge (RLG), systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a guided wave transmission line.
The probe of a guided wave radar system may be a flexible probe such as a transmission line probe, a cable or a wire type probe.
A probe is generally arranged to extend vertically from the top of the tank towards the bottom of the tank. The probe may also be arranged in a still-pipe, which is a pipe or tube arranged within in the tank and in fluid connection with the inside of the tank. A still pipe may for example be preferable if the surface of the product is turbulent, the still pipe may then provide a calmer surface for level measurements.
Furthermore, if the probe is arranged in such a measurement pipe, it is important to ensure that the probe does not make contact with the walls of the tube, which may cause disturbances in the measurement. Therefore, the probe may for example be attached also to the bottom of the tank to prevent lateral movement of the probe within the pipe, thereby avoiding contact between the probe and the inner wall of the measurement pipe.
In some applications it may be desirable, or necessary, to arrange the probe in an inclined or curved measurement tube. For example, the location of the tank may prevent direct access to the roof of the tank, thereby requiring an inclined or curved pipe through which the probe must be arranged in order to gain access to the product to be gauged. The tank may also be formed so that there is no straight path from a transceiver of a guided wave radar system to the bottom of the tank.
However, it would not be possible to use a wire probe in an inclined or curved pipe without modifications as the wire probe would make contact with the inner wall of the pipe, thereby adversely affecting the signal propagation properties of the probe. In particular, a conductive element such as a metal pipe will give rise to reflections of the signal propagating on the probe if the probe is too close to the pipe.
A solution to this problem is suggested by U.S. Pat. No. 7,392,699 where a probe, here in the form of a rod, arranged in an inclined and/or curved pipe is provided with spacers for maintaining the probe in a fixed position in the center of the tube.
However, in the approach disclosed by U.S. Pat. No. 7,392,699, the probe is attached to the spacers, which in turn are attached to the walls of the tube. Such an arrangement is complicated both to install and to remove as the entire measurement tube must be disassembled to remove the probe.